If there is no quad term, QRAO causes ValueError

[Jacomichi]

What is problem

When we try to convert Ising Hamiltonian which has only linear term to QRAO Hamiltonian, it gives ValueError
The error message is as below.

ValueError: max() arg is an empty sequence

How to reproduce

ising = IsingModel({}, {0:1.0,1:1.0,2: 5.0, 3: 2.0}, 6.0)
    max_color_group_size = 3
    _, color_group = greedy_graph_coloring(
        ising.quad.keys(), max_color_group_size=max_color_group_size
    )
    color_group = check_linear_term(
        color_group, ising.linear.keys(), max_color_group_size
    )

    qrac_hamiltonian, offset, encoding = jtqp.qrao.qrac31_encode_ising(
        ising, color_group
    )

[Jacomichi]

This issue is fixed at #36.